42 INTRODUCTION. 



tinguisliecl by the letters pr, and the depressed areas by r / 

 the posterior valves of the operculum are marked p. o., and 

 the anterior a. o. The basal valve (fig. 88) belongs to a 

 Balanus. Fig. 89 is an Acasta^ the cup-shaped base of which 

 is represented at fig. 90. 



In the foregoing explanations we have omitted many of 

 those general terms which^ relating to external characters^ are 

 applicable to shells in almost every division of the system. It 

 may be as well^ however, to enumerate a few of them in this 

 place, although they are explained under their respective 

 letters in the alphabetical part of the work. 



When bars or ribs, or large strise are crossed by others 

 radiating from the umbones, shells are said to be cancellated, 

 as represented in cut, fig. 91. When there is a series of 

 nodules or spines on the upper part of the whorls, they are 

 coronated) as shewn in cut, fig. 92. When a series of project- 

 ing parts overlay each other, in the manner of tiles, as in the 

 cut, fig. 93, the word imbricated is applied. When charac- 

 terized by a regular series of ridges, radiating from the apex, 

 they are pectinated; the species of Chiton, a single valve of 

 which is represented in cut, fig. 94, has received the specific 

 name of pectinatus, in consequence of this character. Shells 

 are said to be plicated when characterized by angular bendings 

 or foldings in their surface, as shewn in cut, fig. 95. A re- 

 markable instance of this is seen in the Ostrsea Crista-Galli. 

 When the margin of any shell has a series of minute notches, 

 resembling the teeth of a saw, it is said to be serrated ; when 

 covered with raised points or spines it is aculeated ; and when 

 striated in both directions, it is decussated ; when covered with 

 a number of raised rounded points, it is gramdated; and having 

 a series of these points placed in a row, near or upon the 

 edge, it is denticulated, as already explained in reference to the 

 outer lips of Spiral Univalves. When the external surface is 



